For controlling the gear ratio change processes in a semi-automatic transmission, besides the driving speed v_F of the motor vehicle, the engine rotational speed of the drive engine and the power demanded by the driver, the driving resistance F_Fw is an essential input parameter. In a moving motor vehicle in each case the shifting speed and the target speed of the next gearshift, i.e. the timing and the target gear of the gearshift, and when the motor vehicle is at rest the respective starting gear, are determined as a function of the driving resistance at the time.
When the drive-train is closed and the service brake is not actuated, in a known manner the driving resistance F_Fw can be determined from the traction or thrust force provided by the drive motor F_Rad that acts on the drive wheels and the acceleration resistance F_B=m_Fzg*a_F calculated as the product of the vehicle's mass m_Fzg and the driving acceleration a_F of the motor vehicle, from the formula F_Fw=F_Rad−F_B. The traction or thrust force F_Rad acting on the drive wheels can for example be calculated from the engine torque of the drive engine, which can be read out of the engine control unit or the CAN databus, or from the torque acting at the output shaft of the transmission, which can be determined by a torque sensor, taking into account the gear ratio at the time and the transmission efficiency in the drive-train. The driving acceleration a_F can be determined by differentiating the driving speed v_F of the motor vehicle, detected by wheel rotational speed sensors, or it can be read out of the CAN databus.
However, when the drive-train is at least partially open, as when maneuvering with a slipping starting clutch, and/or when the service brakes are actuated by a the driver or by a control unit that acts on a service brake actuator, the driving resistance F_Fw can only be determined with sufficient accuracy as the sum of its components (F_Fw=F_Luft+F_Roll+F_Steig), i.e. the air resistance F_Luft, the rolling resistance F_Roll and the inclination-related resistance F_Steig. In such a case the inclination-related resistance F_Steig is obtained as the product of the vehicle's mass m_Fzg, the acceleration g due to gravity, and the value of the road inclination α_Fb, in accordance with the equation F_Steig=m_Fzg*g*sin (α_Fb).
However, besides controlling gearshifts in a semi-automatic transmission, other control functions as well can be carried out as functions of the road inclination. For example, DE 198 38 970 A1 describes a method for starting on a hill for a motor vehicle with an electronic torque control system, in which backward rolling of the motor vehicle during a starting process is prevented in that the torque demand, which depends on the position of the accelerator pedal, is increased by an offset value which is determined proportionally to the road inclination and the mass of the vehicle.
In a method for controlling a rolling lock of a motor vehicle with an electronic brake control system assisted by an external force according to DE 103 03 590 A1, it is provided that when the vehicle is at rest, without actuating the brake pedal, a rolling lock pressure in the wheel brakes is regulated, which is determined proportionally to the road inclination and a further magnitude derived from the mass of the vehicle.
To determine the road inclination it is usual for an inclination sensor that detects the longitudinal inclination of the vehicle to be arranged in the motor vehicle, which is in most cases integrated in the transmission control unit of the transmission. Such an inclination sensor can be designed in accordance with various working principles. For example, from DE 36 34 244 A1 an optical-electronic inclination sensor is known, in which a light-refracting or light-reflecting liquid in a container is arranged in the ray path between a radiation emitter and a radiation detector. DE 40 25 184 C2 describes a capacitative inclination sensor in which an electrically conductive liquid in a container is arranged between the electrodes of a condenser, which are provided with an insulating layer. Finally, from DE 197 52 439 C2 a micro-mechanical inclination sensor is known, in which a mass plate is connected, via at least two pressure sensor units, to a carrier plate which also contains the electronic evaluation unit for determining the inclination.
Since in an arrangement integrated in the transmission control unit the inclination sensor is in fixed connection with the transmission, the orientation of the inclination sensor corresponds to the position in which the transmission has been fitted, which is not always horizontal but can easily be tilted a few degrees away from the horizontal. In such a case, without a zero-point correction the inclination sensor will indicate an upward or downward gradient of the road even though the motor vehicle concerned is actually on level ground. Furthermore, inclination sensors are subject to aging effects which can also lead to indication of an upward or downward road inclination even though the vehicle concerned is on level ground. It is therefore necessary, when the motor vehicle is first used, i.e. after its production, and when it is first re-used, for example after a repair job or replacement of the transmission in a workshop, and even during driving operation of the motor vehicle and the transmission, to calibrate the inclination sensor by determining a current offset value x_Offset for the zero-point correction of the inclination sensor, by means of which the sensor signal x_Nsn of the inclination sensor can then be corrected in relation to the horizontal position of the vehicle (α_Fb=f(x_Nsn−x_Offset)).
In principle, such calibration can be initiated manually by a driver if, when driving on a level, i.e. horizontal road he actuates an operating element, such as a particular function key or the selector lever, in a prescribed manner. Whereas, when the motor vehicle is first used or first re-used, this should be possible without problems for a trained technician driving along a level test stretch, if the calibration function is manually initiated by a driver driving during normal driving operation there is a risk that a slight upward or downward road inclination will be erroneously perceived by the driver as horizontal, or that in the act of initiating and monitoring the calibration function the driver will be distracted in a safety-relevant manner from watching the traffic situation, or that initiation of the calibration function at the appropriate time will be forgotten by the driver and will therefore be omitted.